60 INHERITANCE IN SILKWORMS, I 



test the inheritance possibihties of these variations. They were found 

 to possess no special potency in transmission. 



Fertility as Affected by Age of the Germ Cells. 



The theories of possible species differentiation on a basis of some 

 sort of genetic or reproductive selection (Pearson) or reproductive 

 divergence (Vernon) assume that without actual topographic isola- 

 tion gradual differentiation within a species can come about through 

 discriminate breeding or differences in fertility dependent on the asso- 

 ciation of sexual attraction or antipathy or actual degree of fertility 

 with some other structural or physiological character in the individuals. 

 It has been often suggested that such a relation may exist between age 

 of the germ cells and degree of fertility. I have made a few observa- 

 tions in this connection. 



When the silkworm moth issues from the cocoon it is sexually 

 mature. Mating can take place and often does within a half hour after 

 emergence and the results of this union are fertile eggs. The moths 

 live usually for about three or four days after emergence, at the most 

 but six or seven, so that the age of the moth, and accordingly of the 

 germ cells in functionally active condition, should be reckoned by hours. 

 Matings between moths of exactly known but differing ages were made. 

 For example, males not over four hours old were mated with females 

 as old as fifty-two hours, and with others not over four hours old. 

 Males fifty-six hours old were mated with females just issued and 

 with other females much older. And so on. The eggs from the matings 

 were counted and after the development of the eggs had proceeded for 

 some months the eggs were again counted to the end of determining 

 how many were developing and how many were not. 



The results of the experiments show that eggs from parents in 

 which the male is old did not develop as well as eggs from other par- 

 ents. That is, the extreme age of the female (egg cells) seems to make 

 no difference in regard to the developing power of the fertilized eggs. 

 But the age of the male (sperm cells) does seem to affect the fertility 

 of the eggs. Very old males (sperm cells) seem to be less potent than 

 younger ones. 



